A conventional example of an acceleration detecting element is constituted by a bimorph element of a double end fixing type. For example, a conventional example of an acceleration sensor 20a, shown in FIG. 11 in a simplified manner, includes a bimorph element 1 serving as an acceleration detecting element and an insulating case 2 containing the element positioned therein. The case is fixedly attached on a sensor attaching surface 3 such as a circuit board.
The bimorph element 1 is formed in a rectangular plate shape and is integrated by laminating two piezoelectric ceramics plates 6. Each plate 6 has a signal electrode 4 and an intermediate electrode 5 formed on its top and bottom faces, respectively. The piezoelectric ceramics plates 6 are bonded to each other via the intermediate electrode 5 and are polarized along their thickness direction, each plate being polarized in a direction opposite to that of the other piezoelectric ceramics plate 6. The broken line arrow marks in FIG. 11 designate the directions of polarization. The respective signal electrodes 4 in this example are formed along the longitudinal direction of the respective piezoelectric ceramics plates 6 and are extended to both opposite end portions of each plate.
The insulating case 2 is constituted by a pair of clamp frames 7 having a channel-like shape when seen in a plan view, clamping together both longitudinal end portions in the thickness direction of the bimorph element 1; and also by a pair of case lids 8 enclosing open faces formed by the bimorph element 1 and the clamp frames 7 arranged on opposite sides of the element. The respective signal electrodes 4 of the bimorph element 1 contained in the insulating case 2 are connected to external electrodes (not shown) formed at a pair of opposite outer end faces of the insulating case 2.
An outer surface of the clamp frames 7 or the case lids 8 constituting the insulating case 2 is positioned and fixed on the sensor attaching surface 3 thereby attaching the acceleration sensor. The respective signal electrodes 4 of the bimorph element 1 are connected to wiring patterns (not shown) on the sensor attaching surface 3 via the external electrodes formed on the insulating case 2. These wiring patterns are connected to a signal processing circuit (not shown). The signal processing circuit detects acceleration caused by impact by processing electric signals outputted from the acceleration sensor.
FIG. 12 shows another conventional example of such an acceleration detecting element which is different from the conventional example of FIG. 11 in respect of its polarization. FIG. 12 illustrates electrodes and the like in more detail than in the acceleration detecting element in FIG. 11.
The acceleration sensor 20b includes piezoelectric ceramics bodies 23 in a rectangular plate shape on the main surfaces of which signal output electrodes 21 are formed and wherein an inner electrode 22 in parallel with the signal output electrodes 21 is embedded. Each of the signal output electrodes 21 is constituted by three surface electrodes 24 arranged separately at a center location and end locations along the longitudinal direction of the piezoelectric ceramics bodies 23 and a connecting electrode 25 covering parts of all three surface electrodes 24.
An acceleration detecting element is constituted by the signal output electrodes 21 and the piezoelectric ceramics bodies 23.
One side electrode 24 of the signal output electrodes 21 (one of those on the top side in FIG. 12) is extended to one outer end surface (on the left side in FIG. 12) of the piezoelectric ceramics bodies 23. Also, one of the signal output electrodes 21 on the other side (the bottom side in FIG. 12) is extended to the other outer end surface (on the right side in FIG. 12). Further, ceramics regions 26 and 27 constituting the piezoelectric ceramics bodies 23, which oppose each other on opposite sides of the inner electrode 22, are respectively divided into three portions in the longitudinal direction, namely, center portions 26a and 27a and end portions 26b and 27b, the center portions being divided from the end portions via boundaries where stresses caused by the operation of acceleration are changed. The center portions 26a and 27a and the end portions 26b and 27b are polarized in the thickness direction with senses different from each other by a polarization process using the inner electrode 22 and the surface electrodes 24.
More specifically, the center portion 26a and the left and right end portions 26b constituting the ceramics region 26 are provided with senses of polarization which are different from each other, as indicated by the arrows H and I. Likewise, the center portion 27a and the left and right end portions 27b constituting the ceramics region 27 are provided with senses J and K of polarization which are different from each other, as indicated by the arrows J and K. Further, in this case, for example, the senses of polarization H and J of the center portions 26a and 27a are inward senses wherein the senses are directed toward each other, and the senses of polarization I and K of the end portions 26b and 27b are outward senses wherein the senses are directed apart from each other.
Both edges in the longitudinal direction of the acceleration sensor 20b are fixedly supported by a pair of clamp frames 28 having a channel-like shape when seen in a side view. The respective signal output electrodes 21 formed on main surfaces of the piezoelectric ceramics bodies 23 are connected to external output electrodes 29 and 30 formed on different outer end surfaces of the piezoelectric ceramics bodies 23 and the clamp frames 28.
The acceleration sensor 20b having such a structure operates as follows. When acceleration operates on the acceleration sensor 20b, which includes the acceleration detecting element constituted by the signal output electrodes 21 and the piezoelectric ceramics bodies 23, the center portions 26a and 27a and the end portions 26b and 27b in the ceramics regions 26 and 27 constituting the piezoelectric ceramics bodies 23 are deformed by the operation of inertial force. In this case the respective portions 26a, 27a, 26b and 27b receive tensile stresses or compressive stresses caused by the deformation. In the respective portions 26a, 27a, 26b and 27b an amount of charge generation is enhanced by a synergistic effect of the respective senses of polarization H through K and the received stresses, and an amount of charge generation of the overall acceleration sensor 20b is enhanced, which promotes the detection sensitivity of the acceleration sensor.
In the acceleration sensor 20a or 20b, a maximum electric signal is outputted when acceleration operates in a direction orthogonal to the surface of the piezoelectric ceramics plate 6 or 23, that is, in the thickness direction. Further, an electric signal having the same maximum absolute value with an inverse plus/minus sign is outputted when acceleration operates in a sense inverse thereto, that is, rotated by 180.degree.. In these cases, the direction of the operation of acceleration is in the direction causing the maximum sensitivity, that is, the maximum sensitivity direction P, which is called a main axis of the acceleration sensor. No electric signal is outputted when acceleration operates in a direction tangential to the surface of the piezoelectric ceramics plates 6 or 23 in the acceleration sensor 20a or 20b and accordingly, the detection sensitivity is nullified. Meanwhile, when acceleration operates in a direction between the orthogonal direction and the tangential direction, a detection sensitivity has a value corresponding to an angle .theta. defined by the maximum sensitivity direction P and the operational direction of acceleration, that is, the detection sensitivity has a value of the maximum sensitivity S.times.cos .theta..
When the acceleration sensor 20a having the above-mentioned conventional structure is attached on the sensor attaching surface 3 as shown in FIG. 11, the maximum sensitivity direction P is parallel with or orthogonal to the sensor attaching surface 3. As shown in FIG. 11 two-dimensional orthogonal coordinate axes (plane coordinate axes) x and y are defined on the sensor attaching surface 3. Three-dimensional orthogonal coordinate axes (space coordinate axes) x, y and z are defined with the sensor attaching surface 3 defined as the x-y plane, in a case where one of the case lids 8 of the insulating case 2, integrated with the bimorph element 1, is mounted on the sensor attaching surface 3. In a case where the longitudinally disposed maximum sensitivity direction P of the acceleration sensor 20a is oriented in the direction of the y axis on the sensor attaching surface 3, then acceleration along the x axis or the z axis, that is, acceleration operating in any direction in the x-z plane, cannot be detected.
Further, although not illustrated, if one of the clamp frames 7 of the insulating case 2 is attached on the sensor attaching surface 3 where the orthogonal coordinate axes x and y are defined, and the maximum sensitivity direction P of the acceleration sensor 20a is oriented in the direction of the z axis orthogonal to the sensor attaching surface 3, acceleration in any direction in the x-y plane constituted by the x axis and the y axis cannot be detected.
Therefore, in order to detect all accelerations operating in the respective directions of the mutually orthogonal coordinate axes x, y and z, three acceleration sensors, with their maximum sensitivity directions P aimed in the respective directions of the x axis, y axis and z axis, must be attached on the sensor attaching surface 3. This necessity increases the number of acceleration sensor elements and the installation space, giving rise to high cost and a complicated signal processing circuit for processing electric signals outputted from the three acceleration sensors.
Further, the same problem is naturally caused when the acceleration sensor 20b in FIG. 12 is attached on the sensor attaching surface 3 in place of the acceleration sensor 20a.
To avoid such inconvenience there has been proposed an acceleration detecting element capable of detecting acceleration operating in the three directions of the orthogonal coordinate axes x, y and z by previously inclining the maximum sensitivity direction of the acceleration detecting element upwardly from the sensor attaching surface. Although not illustrated, such an acceleration detecting element is disclosed in Japanese Unexamined Patent Publication No. 133974/1993 wherein the maximum sensitivity direction of an acceleration detecting element having a rectangular plate shape is inclined from a sensor attaching surface by 45.degree. and an edge line of the acceleration detecting element is further inclined from an edge line of an element attaching substrate by 45.degree.. When the acceleration detecting element having such a structure in the acceleration sensor is adopted, accelerations operating in the directions of the x axis, y axis and z axis (hereinafter, three directions in conformity with those shown in FIG. 11) can reliably be detected by a single element.
However, even if accelerations operating in the directions of the three orthogonal coordinate axes x, y and z may be detected, not all accelerations in all directions can be detected. It is impossible to detect acceleration operating in a plane orthogonal to the maximum sensitivity direction. Still further, although the maximum sensitivity direction is naturally inclined from the z axis by 45.degree. in adopting the above structure, the directions of x axis and y axis in this case are inclined from the maximum sensitivity direction substantially by 60.degree. and accordingly, the detection sensitivities in the direction of the x axis, y axis and z axis are not substantially the same.